JP3183788B2 - Halftone area determination device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dot area judging apparatus for reading an original image and judging whether or not the read original image is an original image of a halftone area.

[0002]

2. Description of the Related Art A color digital copying machine reads a color original by a scanner constituted by a CCD (Charge Coupled Device) or the like and converts it into red (R), green (G), and blue (B) electric signals. , Manuscript area as text / line drawing area, halftone dot
It has a function of determining whether the area is a (screen) area or a photograph area, and performing image processing according to the type of the area.

In a character / line image area, edge enhancement and black character enhancement are performed, and in a halftone area, smoothing and blur processing are performed. The halftone dot area is determined as follows. Attention is paid to each pixel in the digital image information that has been read and temporarily stored in the original, and in the detection area of a fixed number of continuous pixels (for example, 3 pixels × 3 pixels) around the pixel of interest, the pixel of interest It is determined whether the density is higher than the density or lower than the surroundings. If the density is higher than the surroundings, this pixel is set as a peak pixel, and if the density is lower, this pixel is set as a dip pixel.

[0004] Then, the peak /
It is determined whether or not the area is a halftone dot area by checking whether the interval between dips is appropriate, or whether or not the appearance pitch of the peak pair and the dip pair is constant.

[0005]

By the way, the color density (gradation) of the halftone dot region is various from high gradation to halftone and low gradation. In the halftone dot region of high gradation, it is more convenient to detect dip pixels in which the peaks are gentle and valley-shaped, and in the dot region of low gradation, the dot candidate dot of the halftone dot is It is more convenient to detect a sharp peak-shaped peak pixel. In the halftone halftone dot area, halftone dot candidate points may be detected by either peak pixels or dip pixels.

If the detection is performed while ignoring this property and fixing either the peak pixel or the dip pixel as in the past, accurate detection may not be possible. Therefore,
The present invention focuses on the gradation of the halftone dot area, and can reliably detect peak / dip pixels regardless of the level of the halftone level of the halftone dot area. It is an object of the present invention to provide a halftone dot region determination device that can be set as follows.

[0007]

According to a first aspect of the present invention, there is provided a halftone dot determining apparatus for determining the gradation of a document image area in three stages based on read document image data. When it is determined that the pixel is in the grayscale region, the dip pixel is detected. When it is determined that the pixel is in the low grayscale region, the peak pixel is detected. The halftone dot area is determined based on the density of the detected peak pixel or dip pixel.

[0008]

In the low gradation area, since the halftone dots are composed of small dots, the peaks of the dots are sharp and the bottoms of the dots are gentle when focusing on the density. In a high gradation area, since halftone dots are composed of large dots, the valleys of density are narrow and deep, and the peaks are wide and gentle. In the halftone area, both the peak and the dip have the same sharpness.

Therefore, according to the present invention, attention is paid to either or both of the peak pixel and the dip pixel according to the gradation of the image area, and the density of the detected peak pixel or the dip pixel is determined. The halftone dot area is determined based on this.

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 is a block diagram showing an electrical configuration of a main part of the color digital copying machine. In this color digital copying machine, a color original is read in one direction (referred to as a main scanning direction) by a scanner 1 constituted by a CCD (Charge Coupled Device) or the like, and is read by yellow (Y), magenta (M), and cyan (C). Convert to primary color signal. The resolution of the scanner 1 is about 400 pixels per inch.

The Y, M, and C primary color signals are subjected to a correction process in the input processing circuit 2 in accordance with the characteristics of the toner and the like.
The information is temporarily stored in the FIFO memory 3, and only information of a certain line (for example, three lines) selected for document type determination is transferred to the character / halftone / photo determination circuit 4 and stored in the line memory 41. It is determined whether the document area is a character / line drawing area, a halftone (screen) area, or a photograph area.

On the other hand, the three primary color signals of Y, M, and C are supplied to a black generation circuit 5, where black (BK) data for compensating for insufficient density of a high-density document is created. Further, the four signals Y, M, C, and BK are extracted one by one by a color selector circuit 6 and converted into signal contents corresponding to a set magnification and the like by a zoom / movement circuit 7.

The filter circuit 8 performs smoothing or edge sharpening in accordance with the type of the document whose area has been determined. Finally, halftone display (dither) is performed by the gradation processing circuit 9 and output to the laser printer through the output control circuit 10. The outline of the signal flow in the color digital copying machine has been described above. Hereinafter, the determination function of the halftone area in the character / halftone / photo determination circuit 4 will be described in detail.

The character / halftone / photo determination circuit 4 first averages the densities of the pixel of interest and its surrounding pixels, and compares the average with a set threshold value. For example, a matrix of 9 dots × 6 lines (this area is defined as one block) as shown in FIG.
VE, if AVE> TH _h (TH _h is a threshold for high gradation detection), the block is high gradation and AVE <TH
_l (TH _l is for low tone detection threshold), then low tone,
Determines that the TH _l <AVE <TH _h if halftone. Here the threshold value TH _l, TH _h may be set to best divide value and each tone. For example, if the level of the image data is 256 tones from 0 to 255, it is simply 3
May be selected, or may be set in consideration of the nonlinearity of the input characteristics of the scanner 1.

The information temporarily stored in the line memory 41 of the character / halftone / photo determination circuit 4 is multiplied by a matrix of 3 dots.times.3 lines (see FIG. 3) to detect peak / dip pixels. . The determination criterion for peak / dip pixel detection is that the target pixel X is determined to be a peak when the value obtained by subtracting the maximum value of the density of the surrounding pixels A to H from the density of the central target pixel X is larger than the threshold value. The pixel X is determined as a pixel, and the pixel X of interest
If the value obtained by subtracting the density of the target pixel X is larger than the threshold value, the target pixel X is determined to be a dip pixel.

Here, the threshold value is set to a value of 5 for all 256 gradations, for example. When the uniform gradation portion of the original is read, when the photograph region and the halftone dot region are compared, the gradation change is hardly 5 or more in the photograph region, and the gradation change is 5 or more in the halftone region. This is because a level difference naturally occurs. The above processing is performed for each dot (pixel of interest) in the middle line temporarily stored in the line memory 41.

On the other hand, it is necessary to determine whether to use the peak pixel or the dip pixel determined by the above method. This determination is made according to the determination result of the high gradation, the low gradation, and the halftone described above. That is, in the case of a pixel in the low gradation block, since a halftone dot is composed of small dots, the peak is sharp and the valley bottom is gentle as shown in FIG. Can not detect the exact position of. Therefore, attention is paid exclusively to the peak pixels, and when the number of peaks in the block is larger than a predetermined number, it is determined to be a halftone dot area.

In the case of a pixel in a high gradation block, since a halftone dot is composed of a large dot, the valley is deep and the peak is gentle as shown in FIG. Can not detect the exact position of. Therefore, attention is paid exclusively to dip pixels, and a block area is determined to be a halftone dot area when the number of dips in a block is larger than a predetermined number. In the halftone block, both the peak and the dip have the same sharpness, and it is not possible to determine which of the peak pixel and the dip pixel is suitable. Therefore, attention is paid to both the peak pixel and the dip pixel, and the halftone dot area is determined based on the logical sum of the number of peaks in the block being larger than the predetermined number or the number of dips in the block being larger than the predetermined number.

When the above processing is completed for the line temporarily stored in the line memory 41, the same is performed for the next line. As described above, the detection by the peak pixel and the detection by the dip pixel are combined in accordance with the average gradation of the area to determine whether or not the area is a halftone area. Therefore, the determination of the halftone area is more reliably performed. be able to.

The present invention is not limited to the above embodiment. In the above embodiment, a block (see FIG. 2) composed of 6 × 9 = 54 pixels is assumed as an area for determining the gradation of an image. However, the present invention is not limited to this.
A block of any size may be assumed. Further, for the detection of the peak / dip pixels, the density of the target pixel X was compared with the density of the surrounding pixels A to H using a matrix of 3 dots × 3 lines (see FIG. 3). However, as another method, for example, a peak / dip pixel can be detected by examining a density relationship between a target pixel and an adjacent pixel based on pixel information of only one line.

[0021]

As described above, according to the halftone dot region judging device of the present invention, one or both of the peak pixel and the dip pixel are focused on according to the gradation of the image region. Since the determination is performed, the peak / dip pixel can be reliably detected regardless of the level of the gradation of the halftone dot region, and the determination of the halftone dot region can be made more accurate.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an electrical configuration of a main part of a color digital copying machine.

FIG. 2 is a diagram showing a 9 × 6 matrix area set for determining a density gradation of an image area.

FIG. 3 is a diagram showing a 3 × 3 matrix area for detecting whether a pixel of interest is a peak / dip pixel.

FIG. 4 is a diagram showing a distribution of peaks constituting halftone dots in a low gradation area.

FIG. 5 is a diagram showing a distribution of dips forming halftone dots in a high gradation area.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Scanner 2 Input processing circuit 4 Character / halftone / photo determination circuit 41 Line memory

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-115987 (JP, A) JP-A-2-115988 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 1/40 G06T 7/40 100

Claims (1)

(57) [Claims] A converting means for reading an original image and converting the read image into image data corresponding to the density; a setting means for setting a detection area of a fixed number of pixels around a pixel of interest; A halftone dot area determining apparatus that detects peak / dip pixels based on density information of pixel data in the area and determines whether the read original image is an original image of a halftone area. The image processing apparatus further includes a tone determination unit that determines a tone of the document image area based on the read document image data, wherein the determination unit determines a dip pixel when the tone determination unit determines that the area is a high tone area. The peak pixel is detected when it is determined to be in the low gradation area, and both the peak pixel and the dip pixel are detected when it is determined to be in the middle gradation area. Halftone dot area determination apparatus, characterized in that for determining the halftone dot region based on the density of the click pixel or dip pixel.